Problem: Luis starts counting at 27, and he counts by threes. If 27 is the 1st number that Luis counts. what is the 9th number that he counts?
Answer: What is the first number that he counts? $27$ What is the second number that he counts? $\begin{align*}&27 + 3 \\ &= 30\end{align*}$ What is the third number that he counts? $\begin{align*}&27 + 3 + 3 \\ &= 27 + (2 \times 3) \\ &= 33\end{align*}$ What is the 9th number that he counts? $\begin{align*}&27 + (8\times3) \\ &= 27 + 24 \\ &= 51\end{align*}$